The invention is based on a hydrostatic drive system for an injection molding machine, the closing unit of which has a movable mold-closing plate which can be moved by a differential hydraulic cylinder.
Various types of drive systems for the movable mold-closing plate of the closing unit of an injection molding machine are known.
Many publications show injection molding machines in which the drive system for the movable mold-closing plate comprises a rotary motor. Thus, for instance, EP 0 167 631 B1 shows an injection molding machine having an electric servomotor which drives a recirculating ball bush in a rotational manner via a pinion, a spindle which is firmly connected to the mold-closing plate extending through the recirculating ball bush. When the electric motor drives the recirculating ball bush, which is fixed in the direction of movement of the mold-closing plate, the spindle together with the mold-closing plate shifts in the axial direction. In an injection molding machine disclosed by EP 0 427 438 A1, an electric motor, via a plurality of gearing stages, drives a pinion meshing with a toothed rack. The toothed rack is firmly connected to the movable mold-closing plate and moves the latter, depending on the direction of rotation of the electric motor, in the closing direction or opening direction of the tool.
A drive system having a rotating hydraulic motor and intended for an injection molding machine has been disclosed by DE 37 18 106 A1. In this publication, a ball screw arranged in a fixed position in the axial direction is driven in a rotating manner via a first gear, which is firmly seated on the ball screw, a toothed belt and a second gear, which is fastened to the output shaft of a hydraulic motor. This rotating movement is converted via a recirculating ball bush into a linear movement of the movable mold-closing plate. A hydraulic motor is of much smaller construction than an electric motor at the same capacity, so that a compact machine construction is possible.
EP 0 135 652 A1 shows a hydrostatic drive system for an injection molding machine in which the movable mold-closing plate is moved by a differential hydraulic cylinder, that is by a hydraulic linear motor. The pressure spaces of the hydraulic cylinder are connected to two consumer connections of a 4/3-way directional control valve, which are shut off in a center position of the directional control valve and are connected in a lateral working position of the directional control valve to a hydraulic pump or a tank, depending on the direction of movement of the mold-closing plate. Pressure medium flowing through the directional control valve is subjected to throttling there, which entails losses of energy which cannot be utilized, these losses being related to the principle involved in the control of a hydraulic cylinder via a directional control valve.
The aim of the invention, for the movable mold-closing plate of an injection molding machine, is to provide a hydrostatic drive system in which only slight losses of energy which cannot be utilized occur.
This aim is achieved according to the invention by virtue of the fact that pressure medium can be delivered by the hydraulic pump into a pressure network having a hydraulic accumulator, that the first pressure space of the hydraulic cylinder is connected to the pressure network, and that the hydraulic cylinder is controlled via a hydraulic transformer which is located with its primary-side pressure connection at the pressure network and via the secondary-side pressure connection of which pressure medium can be fed to the second pressure space of the hydraulic cylinder or discharged from the second pressure space.
Compared with known hydrostatic drive systems having volumetric-flow coupling between the hydraulic pump and the hydraulic cylinder, the control valve is thus replaced by the direct connection of the pressure space on the piston-rod side to the pressure network and by the hydraulic transformer. Work is therefore carried out without throttling of the pressure-medium flow and without the resulting energy losses. The losses which occur are essentially only those caused by the efficiency of the hydraulic transformer.
In addition, in a hydrostatic drive system according to the invention, the installed power for driving the hydraulic pump, for two reasons, can be kept substantially lower than in a hydrostatic drive having volumetric-flow coupling. On the one hand, it is possible, even during the time when the movable mold-closing plate is stationary, that is when the mold is closed and when the mold is completely open, to drive the hydraulic pump by means of the drive motor and to charge the hydraulic accumulator during said dead time, so that a considerable amount of pressure medium under high pressure is available for the initial acceleration of the mold-closing plate during the closing operation and for the movement of the mold-closing plate during the opening, despite a small output of the drive motor. On the other hand, the kinetic energy of the mold-closing plate during the braking of the same is utilized in order to feed pressure medium into the hydraulic accumulator. Therefore some of the energy invested in the acceleration of the mold-closing plate is recovered during the braking. Only the losses of energy which cannot be utilized again have to be compensated for in each case by the hydraulic pump and the drive motor driving it.
The time during which the mold is closed may differ from that during which the mold is open. The drive motor and the hydraulic pump will in each case be constructed in such a way that, during the shorter dead time in each case, the hydraulic accumulator is charged to the pressure necessary for the following movement of the tool clamping plate.
During the longer dead time, the hydraulic pump can be stopped intermittently or switched to circulation of the pressure medium to the tank. In this case, according to features of the invention, a valve which can prevent a flow of pressure medium from the hydraulic accumulator to the hydraulic pump or to the tank is arranged between the hydraulic accumulator and the hydraulic pump. According to a feature of the invention, the valve is preferably a check valve which has a blocking action from the secondary side toward the hydraulic pump.
According to features of the invention, an electric motor whose rotational speed is variable via a control device is preferably used to drive the hydraulic pump. The hydraulic pump is preferably a constant-delivery pump. It is conceivable to drive the electric motor in each case with such a rotational speed that a pressure which is considered to be especially favorable for the motion cycle of the mold-closing plate prevails at the end of a dead time. However, it appears to be more favorable to operate the hydrostatic drive system according to a method of the invention. Accordingly, the electric motor is in each case operated with a rotational speed of high efficiency independently of the duration of the dead time during which the mold-closing plate rests and which may also vary from molding to molding. Finally, if a pressure which is considered to be favorable and is preset for the motion cycle of the mold-closing plate is achieved in the hydraulic accumulator, the electric motor is switched off. The efficiency losses are then especially low.
Hydrostatic drive systems having a hydraulic transformer between a pressure network and a hydraulic cylinder are generally known per se, so that more detailed explanations concerning their mode of operation are not necessary. In addition, reference is made to DE 32 02 015 C2, WO 97/31185 and the book xe2x80x9cHydrostatische Antriebe mit Sekundxc3xa4rregelungxe2x80x9d [Hydrostatic drives with secondary control], which has appeared in the series xe2x80x9cDer Hydraulik Trainerxe2x80x9d as volume 6, second edition, 1996 and has been published by Mannesmann Rexroth AG. Cylinder controls having a hydraulic transformer at the pressure network are described in particular on pages 143 to 156 of the book.